In the context of next generation wireless networks, with a targeted commercialization time around 2020, both millimeter wave radio and centimeter wave radio are considered to be used for the purpose of establishing access links between the access node (or access point, or base station, or Node B or eNode B/eNB) and the user node (or user equipment/UE, or mobile terminal, or mobile device) within a wireless access network.
There are two special aspects of millimeter wave radio from the perspective of its antenna design. One is that the millimeter wave radio will require less power, due to higher frequency, higher noise power and due to normally wider millimeter wave radio channel bandwidth. On the other hand, millimeter wave radio suffers from lower signal-to-noise ratio. However it can be compensated by higher antenna gains with higher antenna directionality. Because of its smaller wavelength, the antenna size is smaller for millimeter wave radio. The transmitter and receiver of millimeter radio can accommodate more antenna elements hence narrower beams can be produced with large number of antenna elements which will yield higher antenna gains. The beam can be formed through a phase control system such that the direction, as well as the beam width can be adjusted. Narrower beam is beneficial for the purpose of providing higher antenna gains, causing less multi-path fading as well as minimizing cross link interference.
The large available bandwidth and the high gain narrow antenna beam make the millimeter wave link very suitable for providing very high data throughput between the access node and the user node. However, it is difficult to establish and maintain such link between the access node and the user node via narrow beam since the user node is constantly moving and turning. The localization of the user node as well as the determination of the optimal antenna beam direction for both the access node and the user node can benefit from the usage of combined millimeter wave link and centimeter wave link.
US 2014/098912 A1 discloses that initially, the mobile station detects the strongest beam direction for a wide beam for communicating with a base station. Then the mobile station sends a transmitter slice/ID to the base station in the uplink identifying the wide beam slice. The mobile station then switches to narrow beam mode to scan a set of narrower beams within that selected slice and detect the strongest beam direction for a narrow beam. The mobile then sends a slice/ID to the base station in the uplink identifying the narrow beam slice. The mobile station then receives data that is transmitted on the paired narrow beams.
However, the above mentioned method requires a high signaling effort between the mobile station and the base station, which increases latencies of the beam search.